1. Field of the Invention
This invention relates generally to electric plugs and, more particularly, to an electric plug which prevents accidental electric shock when the plug is only partially inserted into an electric socket.
2. Prior Art
Electrical devices, such as lamps, sweepers, kitchen appliances, power tools, television sets, stereos, and many others all have electrical cords extending from them. These cords all carry a terminal plug that is inserted into an electrical outlet to power the devices. Each plug has two or more prongs that are inserted into mating slots in the socket of an insulated receptacle, where they engage electrically-conductive contacts. These contacts not only establish electrical contact with the prongs, but also grip the prongs with sufficient force to prevent accidental withdrawal of the plug.
A problem develops when a plug is only partially inserted into the socket, thereby leaving a gap between plug and receptacle with a portion of both prongs exposed. If the gap is sufficient, two things can occur. First, if there is no electrical contact, the appliance will not operate. Second, if there is electrical contact, someone, usually a child, or something, such as a household, pet, could insert a body part into the gap and touch the exposed portions of both prongs simultaneously. If so, an electric shock will ensue, much to the hazard of the child. This could cause burns or worse, depending on the size of the child. Also, even if only a very slight gap exists between plug and receptacle, too shallow for fingers to intrude, a thin metal object, such as a letter opener, screwdriver, table knife, fork or spoon could be inserted by an inquisitive child resulting in the same potentially-disastrous shocking incident.
Another problem with exposed, electrically-conductive prongs is a danger of house fire. Research has indicated that over 20% of house fires in the U.S. are caused by arcing or shorting or other malfunctioning of electrical plugs.
In recognition of these hazards, and in response to many incidents of such accidental shockings of small children, many attempts have been made to devise a safety plug which reduces the chance of such accidental shockings. Many of these comprise electric plugs having an insulating material installed on the prongs adjacent the plug base. In the case of partial plug insertion, this arrangement leaves only the insulated portion of the prongs exposed, preventing accidental shocking.
In one version, the plug prongs are cut away adjacent the plug base and an insulating material insert is installed, as shown in U.S. Pat. No. 3710287—Eckert, U.S. Pat. No. 3,533,052—Degataeno, U.S. Pat. No. 5,641,311—Chuang, and U.S. Pat. No. 6,109,977—Baxter et al. The problem with this solution to the accidental shocking problem is that the prongs are weakened near their base, resulting in inability to endure repeated flexures, as their plugs are repeatedly inserted into and yanked out of receptacles, often accompanied by bending of the prongs, without fatiguing and breaking. Another problem with this type of construction is the eventual wearing of the insulating material, which will eventually expose the conductive prong base, thus defeating the protection. Also, the protective coating could be accidentally cut, with the same result
Another type of so-called safety pug features the addition of an insulating sleeve on the prongs adjacent their bases, as shown in U.S. Pat. No. 2,226,148—Taylor and U.S. Pat. No. 5,897,398—Maeda. One problem with this construction is that this thickens the prongs so much that insertion and withdrawal are rendered much more difficult, due to the increased friction caused by the added thickness of the prongs which the receptacle contacts must accept. Also, the sleeves could be cut, as above, or pulled off the prongs.
A third proposed solution is shown in U.S. Pat. No. 3,631,320—Eckert in which a flexible, resilient sleeve, boot or bellows is attached to the plug body surrounding the prongs. As the prongs are inserted into a receptacle, the boot engages the receptacle and is compressed between the receptacle and the prongs. This arrangement overcomes the thickness problem of static sleeves, but is an expensive addition to the plug, and it, too, could suffer failure if the boots are cut or wear out, or are pulled off.
Thus, there is a need for a safety plug which will prevent accidental shockings, and which is not prone to failures due to wear, cutting or dislodgement.
There is also a need for a safety plug which will not suffer prong fatigue and breaking.
It would also be advantageous to provide an electrical plug attached to an electrical appliance that indicates failure to fully insert the plug into an electric receptacle by rendering the appliance inoperable.